As explained in our U.S. Pat. No. 4,732,312, which issued on Mar. 22, 1988, the combined use of superplastic forming and diffusion bonding (SPF/DB) offers the potential to manufacture lighter and less expensive aircraft structures than those made by conventional means. It is particularly attractive for sheet metal structures because part and fastener counts could be reduced, thereby significantly decreasing assembly labor. Also the fabrication of structures to near-net shapes using SPF/DB technology can improve material utilization and reduce machining time and costs.
The application of SPF/DB to titanium alloys has been well demonstrated but this is not the case for advanced high strength aluminum alloys. Although impressive SPF behavior has already been demonstrated for aluminum alloys, such as 7475, and work has begun on developing superplastic properties for Al-Li alloys, the diffusion bonding technology for these materials is lagging. A simple and cost-effective diffusion bonding technique compatible with SPF technology could significantly advance the use of aluminum structures.
In diffusion bonding, flattening of the abutting surfaces is necessary in order to achieve intimate interfacial contact. Metals like titanium, which have surface oxides that easily dissolve in the metal during heating, can be readily diffusion bonded without the use of special surface preparations or interlayer diffusion aids. Unlike titanium, aluminum and its alloys (as well as, for example, zirconium and vanadium and their respective alloys) form insoluble oxides which do not readily dissolve during bonding and thus act as barriers to intimate metal-to-metal contact and subsequent diffusion.
Typically, aluminum has been diffusion bonded by methods which rely upon considerable deformation (up to 60 percent) and pressure (up to 40,000 psi) to rupture surface materials to dissolve oxides and aid diffusion. In general, such methods are not compatible with the constraints imposed by SPF technology or the mechanical property requirements of a high performance structure.
For example, practical limitations set by production equipment dictate that SPF pressures probably should be limited to 1,000 psi and perhaps should be much lower. In addition, other variables important to SPF, such as starting microstructure, dwell time, forming rate, dynamic recrystallization, and post heat treatment must be considered.
Aluminum has also been diffusion bonded by removing the surface oxide layers by sputtering or other suitable techniques in a hard vacuum or reduced pressure inert gas environment in order to prevent the oxide layer from being formed again before bonding. However, pressures below 10.sup.-9 Torr must be maintained in order to keep the oxide layer from forming again almost instantly in a hard vacuum and pressures of approximately 10.sup.-6 Torr in an inert gas environment are desirable. In other words, using these techniques, the cleaned surfaces cannot be exposed to air prior to bonding. It is generally believed that cleaning techniques such as abrading, chemical etching or dissolving the oxide by use of fluxes, if carried out in a vacuum or low pressure inert gas environment, to preserve the oxide cleaned surface, present problems in controlling removal of oxides from the work, etching solutions or the process chamber.
In our U.S. Pat. No. 4,732,312, a method is discussed for achieving diffusion bonding of surface layers of an alloy sheet, such as aluminum, having surface oxide coatings of low solubility in the alloy. The discussed method comprises the steps of: treating said alloy so that at least the surface layers to be bonded have a fine grain structure; removing existing surface oxide coatings from the surface layers to be bonded; diffusion bonding the surface layers to one another by placing the alloy to be bonded under a pressure sufficient to cause disruption of the oxide coatings and insufficient to cause macroscopic deformation of the alloy, while heating the alloy in a non-oxidizing atmosphere for a time sufficient for diffusion bonding to occur. Generally, the deformation will approach zero percent or a very low amount on a macroscopic scale. Pressures of less than 1,000 psia and preferably less than 100 psia may be applied to force the surfaces together. The diffusion bonding generally takes place at temperatures below the melting point of the alloy by several degrees centigrade or at the superplastic forming temperature for a time ranging between one and ten hours. At least one part of the diffusion bonded assembly may be superplastically formed to produce a structurally useful component of a predetermined configuration.
More important to the present invention, the method of our mentioned patent may also comprise the step of treating alloy sheets so that the alloy, or at least the surface layers thereof, have a fine grain structure of the type associated with superplastic forming properties. This is done by thermomechanically processing the surface layers of the sheets by heated rollers. Enhanced localized surface deformation of such alloys during bonding resulting from the superplastic microstructure leads to extensive oxide film disruption, thus facilitating bonding.
After diffusion bonding (and superplastic forming) the bonded structure may be further heat treated by solution treating, quenching and aging.
The surfaces to be bonded are prepared by abrading with successively finer grades of grinding paper, rinsing with water, abrading with a metallic brush, and removing the brushings. The abrading may be performed by abrading in a first direction, and abrading in a second direction substantially at right angles to the first direction. The brushings may be removed by exposing the surfaces to a stream of filtered compressed air moving at a velocity sufficiently high to remove the brushings.
Pressure may be applied to the components to be bonded by forcing the surfaces together by placing the components in a fixture, exposing a first opposite surface to a first surface layer to be diffusion bonded to one of a partial vacuum and a pressurized gas and exposing a second opposite surface to a second surface layer to be diffusion bonded to another of said partial vacuum and pressurized gas.
Although the method of our patent achieves success with sheet materials, thermomechanical rolling of thick plates, bars, and irregularly shaped alloy materials is impractical or impossible, thereby preventing surface treatment of these materials to achieve surface fine grain structure of the type associated with superplastic microstructure.